A standard six-high rolling stand of the type used for rolling strip steel has a pair of vertically spaced nip-defining working rolls of relatively small diameter. Respective upper an lower inner backup rolls of larger diameter bear respectively down and up toward the nip on the respective working rolls, and respective upper and lower outer backup rolls of still larger diameter bear toward the nip on the respective inner backup rolls. Thus the bendability of the small-diameter working rolls is largely canceled out by the rigidity of the larger-diameter backup rolls so that the large forces required for rolling can be brought to bear on the workpiece.
Even with such a system the workpiece is normally subjected to somewhat more pressure at its longitudinal edges than at its center so that the working rolls wear more at their portions corresponding to the edges of the strip being rolled. Old German patent 955,131 suggests that this problem can be somewhat alleviated by axially oppositely shifting the two inner backup rolls so that one end of the one working roll and the opposite end of the other working roll are not backed up. This allows some deformation of the working rolls in the corresponding regions and thereby ensures more uniformity of workpiece thickness. Such axial shifting of the inner backup rolls allows the stand to be adjusted for the particular workpiece width exactly to produce a rolled product of relatively uniform thickness. In order further to prevent the workpiece from being left slightly too thick at its edges, the working rolls in this system are somewhat crowned, that is are of somewhat greater diameter at their centers than at their ends.
It is also possible to use perfectly cylindrical working rolls in a system such as described in German patent publication No. 1,281,981. Here the ends of the working rolls are pushed inwardly in a direction bowing these rolls toward the nip. The ends of these working rolls are supported on the stand frame or housing in journal blocks which are urged by actuators with great force toward the nip to achieve this effect.
British patent application No. 678,285 suggests a similar system wherein both the working and inner backup rolls have their ends mounted in journal blocks that can be pushed by appropriate actuators toward the nip so as to bow the ends of the working roll that extend past the workpiece strip edge toward each other. Such an arrangement allows cylindrical working rolls to be employed. In addition the actuators for at least the upper inner backup roll are useful, when double-acting, to lift this roll out of contact with the respective working roll when the working rolls have to be replaced.
It is also known from German patent publication No. 2,752,750 to use axially fixed rolls in a six-high stand wherein each inner backup roll is axially somewhat shorter than the respective working roll, and similarly each outer backup roll is somewhat shorter than the respective inner backup roll. In this arrangement actuators are provided for bending the ends of the inner backup and working rolls inwardly toward the working nip to compensate for their not being supported. Thus the edges of the rolls turned toward the nip are made perfectly straight.
The problem with this last-described system is that it is only usable with strip material whose width lies within a very narrow range, as only the end regions of the rolls can be bent. The minimim workpiece width is therefore the axial length of the central portion of the outer backup rolls and the maximum workpiece width is the axial length of the central portion of the inner backup rolls. This limitation of the size of workpiece for the system is a great disadvantage.